The present invention relates generally to a beam control method and an apparatus for carrying out the same. More particularly, the invention is directed to a beam control method and a beam control apparatus which can be employed for depicting or drawing a pattern on a specimen such as a wafer and the like with an electron beam while suppressing the influence of drift or deviation of the electron beam from a desired or aimed position for irradiation.
With an increasing tendency to use semiconductor devices with a higher integration density, an optical exposure or irradiation apparatus and a pattern depiction apparatus for depicting or forming a fine pattern on a specimen such as a semiconductor wafer, mask or the like is imposed with more and more severe requirements relating to the capability and performance thereof. Of these apparatuses, it is expected that the optical exposure apparatus will encounter great difficulty in its application to the manufacture of semiconductor devices of the next generation. Under the circumstances, an electron beam pattern depicting apparatus adapted for drawing or depicting patterns on semiconductor specimens such as wafers by using an electron beam has been developed as an apparatus to replace the optical exposure apparatus mentioned above.
For a better understanding of the invention, description of an electron beam apparatus known heretofore will be described briefly on the assumption that the apparatus is applied for depiction of a pattern on a mask by reference to FIG. 1 of the accompanying drawings. As is shown in the figure, a mask 22 disposed on an XY-stage 21 is irradiated with an electron beam EB with the aid of an electron optical system 1, whereby a pattern is formed on the mask 22. In this type of electron beam apparatus, the position actually irradiated with the electron beam often drifts from a desired or aimed position which should originally have been irradiated, due to deviation in the position of the specimen 22 disposed on the stage 21. In an attempt to correct or cancel such deviation, a reference mark 23 is usually provided on the XY-stage 21 at a peripheral portion thereof (outside of the range of irradiation with the electron beam EB). With this arrangement, deviation or drift is corrected in a manner described below. After displacement of the XY-stage for establishing positional alignment between the electron beam EB and the reference mark 23, magnitudes of displacement of the XY-stage 21 are measured by a laser type range finding instrument 20 in the X- and Y-directions, respectively. On the basis of the displacement thus measured, magnitude and the direction of the drift in the position of the irradiating electron beam EB on the mask 22 is determined, whereon the irradiating direction of the electron beam EB is changed so that the drift can be eliminated, to thereby allow the electron beam EB to irradiate the mask 22 at the originally aimed position.
In the case of the method of controlling the position of the irradiating electron beam BE by detecting the reference mark 23, as described above by reference to FIG. 1, the stage 21 has to be moved periodically at a predetermined time interval in the course of a pattern depiction for the purpose of establishing positional alignment between the electron beam EB and the mark 23 to thereby correct or adjust the actual position of the irradiating electron beam, which, of course, results in excess time required for the pattern depiction.
In the case of an application where the specimen to be subject to the pattern depiction is a wafer, an alignment mark 34 formed on the wafer 6 is detected in place of the reference mark mentioned above, as shown in FIGS. 2A and 3 of the accompanying drawings. Parenthetically, a reference numeral 7 in FIG. 2A denotes a resist layer. Since the alignment mark 34 is positioned in the vicinity of a circuit pattern 33, the time taken for establishing the positional alignment between the electron beam and the alignment mark 34 can certainly be reduced, which, in turn, means that the time required for the correction to make the actual position irradiated with the electron beam EB and the originally aimed position coincide with each other can be correspondingly reduced. A typical one of such alignment technique is disclosed in Japanese Utility Model Application Laid-Open No. 29953/1981 (JU-A-56-29953) which may be referred to for more particulars.
It is however noted that when a multiplicity of resist layers are provided in a stacked structure, it often becomes difficult or even impossible to detect the alignment mark.
Besides, in the current status of the art, there exists inevitably a deviation 3.sigma..+-.0.2 .mu.m to .+-.0.25 .mu.m between the actually irradiated position and the desired or aimed position (reference may be made to "Current Status of E-Beam Lithography": Bull. Japan Soc. of Prec. Engg., Vol. 22, No. 4, Dec. 1988). Accordingly, even when the alignment mark 34 can be detected, problems remain with respect to the accuracy and hence the capability of the electron beam irradiation apparatus to manufacture IC devices of the next generation. The main factors giving rise to such alignment errors include: (1) fluctuation of the beam impinging position due to vibration of a column constituting an integral part of the electron optical system, (2) variation or change in the beam impinging position under the influence of electric charge stored in a specimen (wafer, mask or the like) as well as electric charge built up internally in an electron beam lens/deflection system, (3) error involved in the detection of the alignment mark, and others. For coping with the electric charge formed in the specimen, such approach has been proposed that an electrically conductive material is contained in the resist layer.